A Borosilicide with Clathrate VIII Structure

The high-pressure phase Na8BxSi46–x (3 < x < 5) is the first representative of a borosilicide crystallizing in the rarely occurring clathrate VIII type structure. Crystals with composition Na8B4Si42 (space group I43̅m; a = 9.7187(2) Å; Pearson symbol cI54) were obtained at 5–8 GPa and 1200 K. The clathrate I modification exists for the same composition at lower pressure with a larger cell volume (Pm3̅n; a = 9. 977(2) Å; cP54). Profound structural adaptions allow for a higher density of the clathrate VIII type than clathrate I, opening up the perspective of obtaining clathrate VIII type compounds as high-pressure forms of clathrate I.


Preparation
2. Crystal Structure of the clathrate VIII phase from electron diffraction data

Preparation
Sample handling was conducted in an argon-filled glove box except for the high-pressure synthesis and the washing procedure of the product. The precursor compound Na4Si4 was synthesized from sodium (Chempur, 99.95%) and silicon (Chempur, 99.9999%) in a closed tantalum tube by annealing at 750 °C for 7 h, followed by slow cooling to room temperature within 8 h. Amorphous boron (Alfa Aesar) was purified and activated in a streaming hydrogen plasma [1]. The educt mixtures with molar ratio Na:B:Si = 5:2:5 were thoroughly ground in agate mortars. The mixture was filled into a boron nitride crucible of 3 mm diameter and placed in MgO octahedra with an edge length of 18 mm. The high-pressure, high-temperature syntheses were performed in a multi-anvil press comprising a Walker-type module [2]. Pressure and temperature were calibrated before the experiments by recording the resistance changes of bismuth and by thermocouple-calibrated runs, respectively. To prepare Na8B4Si42, a pressure of p = 6 ± 1 GPa was applied at room temperature. After heating to T = 950 ± 100 °C within 15 min and annealing for 10 min, the samples were quenched under load. The reaction products were washed with ethanol and deionized water to remove traces of Na4Si4, followed by washing with ethanol and acetone and drying at room temperature.

Crystal structure of the clathrate VIII phase from electron diffraction data
The clathrate VIII structure was first identified by combining conventional transmission electron microscopy, selected area electron diffraction (SAED), and manual selected area electron precession diffraction tomography (SA-PEDT). The investigations were performed on an FEI Tecnai F30-G2 supertwin microscope operating at 300 kV. The microscope was equipped with a CCD camera (GATAN Inc.) and a standard double-tilt holder (GATAN Inc.) with a tilting range of ± 46° of the holder axis and ± 30° perpendicular to it. The SAED mode was used for tomography data collection.
A specimen suitable for the TEM investigations was prepared with the focused ion beam technique (FIB) using a Quanta 200 3D ion/electron dual-beam device (FEI, Eindhoven). The device was equipped with an omniprobe micro-manipulator (W needle) and could be used both as a scanning electron microscope (SEM) and a scanning ion microscope (SIM). SEM images revealed, besides smaller grains, crystalline grains with a cubic habit which were used for the subsequent investigations (Fig. 1a).
Prior to the sample cut, protecting Pt layers (24 µm long, 2 µm thickness, 2 µm high) were deposited on the selected part (parallel and perpendicular to the c axis of microcrystals arrays, Fig. 1a) using an acceleration voltage of 30 kV and a current of 0.1 nA. Each cross-section (2 μm thickness) was then prepared with a Ga-ion beam with an acceleration voltage of 30 kV and a 1-0.5 nA current. The manufactured cut was transferred onto a copper Omniprobe TEM holder using the in-situ lift-out technique [3]. Subsequently, the cross-section was thinned to a thickness of about 60 nm applying an acceleration voltage of 30 kV and currents of 0.5-0.01 nA of the Ga-ion beam.
The lamella contained a silicon-rich phase, later revealed as a clathrate VIII phase (bright areas in Fig.   1b) and a boron-rich phase (dark areas) with approximate composition 'Na2B6Si2'. The lamella (red area, Fig. 1b    clathrate-VIII type of structure. However, strong dynamic multi-beam effects biased the recorded dataset of 447 measured and 212 symmetry-independent reflections so that the structure refinement converged to a high Rgt value of 0.45. To reduce the multi-beam dynamical effects, a SA-PEDT experiment was performed with a precession angle of the electron beam of 1.26°. Intensity data were collected in a tilt sequence from -43° to +43° (step-width of 1°). The 2D-PED data (87 images in dm3 file format) were converted to the TIF format to be indexed and integrated with the PETS software package (version 2.0) [4]. Indexing revealed an I-centred cubic unit-cell (a ≈ 9.9 Å). The electron beam precession and the smaller camera length (300 mm) resulted in a larger data set. 7589 reflections were measured, yielding 922 symmetry-independent reflections stored in standard hkl file format. The 3D-PED diffraction volume is shown in Fig. 4 projected along a*, b*, and c* axes.  1°). The acquisition tilt axis coincides with the TEM goniometer axis ( angle tilting axis). Images and the 3D-PED volume reconstruction were made using the PETS 2.0 software [4].
Crystal structure refinement (Jana2006 software [5]) started with the atomic positions of -Eu8Ga16Ge30 [6]. Unlike the XRPD study, Boron atoms can be assigned to two crystallographic sites, Si3 and Si4, resulting in the composition Na8B8(1)Si38 (X-ray: Na8B4Si42). However, the high residue of Rg = 0.20 (816 reflections with I > 2(I)) caused by multi-beam dynamical effects may bias the structure model. The crystallographic data are listed in Table 1, and the atomic coordinates and displacement parameters are listed in Table 2. Please note that the cif file is based on more accurate single-crystal data ( Table 3,4).

Crystallographic data of the clathrate VIII phase from single-crystal X-ray diffraction data
Structure refinement with Shelx software [7] against F 2 resulted in the composition Na8B4.2(1)Si41.8(1) and a residual value of R(F 2 ) = 0.03 (Table 1). Refinement with WINCSD software [8] against F resulted in the composition Na8B4.2(1)Si41.8 (1). The real value cannot be decided based on our data, and we estimate the accuracy of the composition to Na8B4(1))Si42(1). Tables 3-5 present the results of the structure refinement with Shelx, used in the main text. Alternatively, Table 6 shows a split atom model refined with WINCSD.      Table 7. Crystallographic data of a clathrate I single-crystal Na8B4.1Si41.9 investigated with X-ray diffraction (293 K). Further details on the crystal structure investigations can be obtained from the FachinformationszentrumKarlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (email: crysdata@fizkarlsruhe.de, http:///www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting the depository numbers CSD-2169576.

Magnetic susceptibility
The magnetic susceptibility was measured using a polycrystalline sample of cylindrical shape on a squid magnetometer (MPMS XL-7, Quantum Design) between 1.8 and 350 K in external fields of 15000 to 7000 Oe. Due to the byproduct Na2B6Si2 in the sample, the evaluation of absolute values was unfeasible.
However, the Pauli paramagnetism of the phase mixture is in line with the expected metallic behavior and superconductivity can be ruled out down to 1.9 K.